Process for controlling scale formation in aluminum etching



United States Patent PROCESS FOR CONTROLLING SCALE FORMA- TION IN ALUMINUM ETCHING Walter A. Reissig, Baldwinsville, N. Y., assignor to Allied Chemical & Dye Corporation, New York, N. Y., a corporation of New York No Drawing. Application September 23, 1954, Serial No. 457,993

4 Claims. (Cl. 41-42) This invention relates to a method of etching or dissolving aluminum from masses of aluminum or aluminumbase alloys.

In many instances where articles of aluminum or aluminum-base alloys are fabricated or where aluminum or aluminum-base alloy metals are secured to containers of other materials, it is necessary to etch the aluminum surfaces either to provide a base for further finishing or to give a final decorative finish. In other industrial operations, removal of aluminum-foil or aluminum-alloy-foil labels from bottles or other containers has also been performed to recover the aluminum content. Typical industrial practice in etching or dissolving materials of the above-noted type is to contact the aluminum-containing surface with solutions of etching compound which may preferablybe hot aqueous solutions of alkalies. Typical etching or dissolving alkalies which have been employed include sodium hydroxide, potassium hydroxide, trisodium phosphate, soda ash, or similar materials or mixtures of these chemicals in various proportions. Sodium hydroxide in aqueous solution is the most commonly employed agent, and it reacts with the aluminum present as it dissolves or etches the same to yield sodium aluminate and hydrogen, according to the equation:

Although caustic etching of aluminum has generally been employed, it is found that it is not entirely satisfactory because of certain inherent disadvantages. In particular a precipitate of aluminum hydroxide may be decomposed either immediately under the conditions of operation, or subsequently on standing to yield alumina, A1203, in anhydrous or hydrated form. This precipitate of alumina is undesirable in that it deposits as a hard, adherent, rock-likescale on the inner walls, bottoms, and internals of the reaction vessel. Removal of this scale from the reaction vessel is found to be extremely difficult. Chemical removal as by means of acid, is expensive and subjects the reaction vessel to corrosive conditions which tend to shorten the life thereof. Mechanical removal of scale as by hammers and chippers may result in damage to internals of the vessel.

Among the disadvantages of this process conducted according to prior practice may be noted the difii culty of maintaining the solution at the desired temperature of etching or dissolving, i. e. the deposition of alumina on the coils of internal heat exchangers lowers the coefiicient of heat transfer to such an extent that the heat flux is substantially decreased.

It is an object of this invention to control the character of precipitated alumina in aluminum etching operations whereby the same may be rendered soft, mushy, nonadherent, and easily removable from surfaces on which it deposits.

Other objects will be apparent to those skilled-immeart on inspection of the following description.

2,741,051 Patented Apr. 10, 1956 According to certain aspects of this invention, the treatment of metal surfaces containing aluminum or aluminum alloys may be efiected with aqueous solutions of caustic etching agents typical of which are those which may be selected from the group consisting of sodium hydroxide, potassium hydroxide, and mixtures thereof, containing as dispersing agent admixed therewith a sulfonated organic compound. Preferred dispersing agents include salts, especially alkali-metal salts of high molecular-weight, naturally occurring, organic compounds, which have been sulfonated. Among the dispersing agents which may effect the desired result are those prepared from lignins e. g. sodium lignosulfonate. Typical available sodium lignosulfonate prepared from naturally-occurring sources may contain non-reducing carbohydrates and lesser amounts of other ingredients including inorganic salts. Preferably the dispersing agent may be added to the caustic in amount of 0.5% to 5.0%, say 1.0% to 2.0% by weight thereof to satisfactorily dissolve or etch the surface and to permit precipitation of the desired soft,

' mushy, non-adherent sludge.

- composition containing from 95.0% to 99.5%, say 98.0%

of a caustic etchingagent typical of which is one selected from the group consisting of sodium hydroxide, potassium hydroxide, and mixtures thereof, and as dispersing agent a sulfonated organic dispersing agent in amount of 0.5%

, to 5.0%, say 2.0% by weight of caustic. Preferred agents 1 ipersing agent with'an alkali. When caustic soda is the' include salts, especially alkali-metal salts of high molecular weight, naturally-occurring, organic acids or other compounds, which have been sulfonated. Among the agents which may effect the desired result are those prepared from lignins, e. g. sodium lignosulfonate. Typical available sodium lignosulfonate prepared from naturally occurring sources may contain non-reducing carbohydrates and lesser amounts of other ingredients including inorganic salts. Preferably the agents may beadded to the caustic in amount of 0.5% to 5.0%, say 1.0% to I 2.0% by weight thereof to satisfactorily dissolve or etch the surface and to permit precipitation of the desired soft, mushy, non-adherent sludge.

The composition may be formed by mixing the disalkali for example, the dispersing agent may be mixed with the solid flake prior to its dissolution, or it may be added to the solution formed by dissolving the caustic in water.

In accordance with one embodiment of this invention, a sheet or strip or other commercially available shape of aluminum or aluminum-containing alloy or material containing an aluminum or an aluminum-containing alloy section to be treated, is immersed in a caustic etching or dissolving bath containing the dispersing agent and an alkaline reagent. Typical alkaline reagents include sodium hydroxide, and potassium hydroxide. Commonly the bath contains commercial caustic soda, sodium hydroxide, and it will be hereinafter referred to for purposes of convenience as containing this ingredient. Caustic solutions may contain from 1.0 to 10.0 parts by weight of caustic per parts by weight of water.

Typical of the sulfonated organic dispersing agents which may be employed as additive whereby the character of the precipitated alumina may be controlled, is sodium lignosulfonate. The trademark Marasperse N designates a product (commercially available as a dark brown powder containing approximately 6.0% moisture) produced by Marathon Corporation, Rothschild, Wisconsin, which is essentially a sodium lignosulfonate, having 3 the following approximate composition on a moisturefree basis:

Percent Sodium lignosulfonate 78.12 Non-reducing carbohydrates 11.55 Organic combined sodium notas sulfonic 3.11 CaSO4.%H2O n.. 1.28 MgSOeHzO 1.58 NaasOi 4.26 Other .10

The term sodium lignosulfonate" is intended to designate the product formed by neutralizing a sulfonated pure or crude lignin (e. g.'waste-from a paper mill from which the cellulose has been removed) and as is well known to those skilled-in-the-art such products frequently contain impurities.

In typical operations the solution of caustic etching or dissolving agent may be formed and the sodium lig'nosulfonate or other sulfonated organic dispersing agent added thereto in desired amount. If the sulfonated organic dispersing agent was mixed with the caustic previously, the mixture may be added to water to form the desired etching or dissolving bath. Typical concentration of dispersing agent, with respect to the caustic present, may range from 0.5% to 5.0%, preferably from 1.0% to 2.0%, say 2.0%. When the sodium lignosulfonate employed is Marasperse N," the preferred amount of'that material present is from 1.0% to 2.0%, say 2.0% of caustic. Typical mixtures of various alkaline materials and dispersing agents, expressed on dry basis, may be as follows:

A. Caustic soda 95.0% to 99.5%, say 980% Marasperse N 5.0% to 0.5%, say 2.0%

B. Caustic potash 95.0% to 99.5%, say 98.0%

Marasperse N 5.0% to 0.5%, say 2.0%

The aluminum may be retained in the bath for l to 15 minutes depending upon the particular etfect desired. Preferred temperature of the bath for'etching is 60 C. to 90 C.; for solution-of aluminum, the preferred temperature is 65 C. to 75 C., say 70 C.

In typical commercial etching operations according to prior practice, wherein a bath containing 800 pounds of caustic soda in 3000 gallons of water (i. e. a 3.1% solution of caustic by weight) is used at temperature of 80 C. it is found that after the bath has been in operation for 15 hours, a precipitate of alumina becomes-noticeable as a deposit on the surfaces of the vessel. When the 4 vessel is cleaned after 40 hours of operation, it is noted that typically the precipitate may be mm. thick. This precipitate is hard, rock-like, and adherent to the surfaces on which it is deposited. It is not readily removable as by fiushing with a stream of water from a hose.

When operating in accordance with this invention, 16 pounds of a sulfonated Organic dispersing agent such as Marasperse N may be added to a solution otherwise identical to the above-noted solution employed to dissolve aluminum labels from glass containers. When this bath is cleaned after 40 hours of operation, the precipitate formed on the internals of the vessel is found to be soft, mushy, and non-adherent. Upon draining the solution from the vessel, the sludge is readily flushed therefrom by means of a low-pressure spray of water from a hose.

Regardless of what the particular chemical, physical, or crystallographic mechanism may be, it has been observed that the deposit formed in accordance with this invention is readily removable from surface on which it rests. It appears however that the sulfonated organic dispersing agent may mechanically intermix with and be entrained by the precipitating alumina to an extent sufficient to permit displacement of the deposited alumina particles from the rigid matrix in which they would otherwise deposit.

1 claim:

1. The process of etching a body of aluminum comprising contacting such a body under etching conditions with an aqueous solution of a caustic etching agent containing sodium lignosulfonate.

2. The process of etching a body of aluminum comprising contacting such a body under etching conditions with an aqueous solution of a caustic etching agent containing sodium lignosulfonate in amount of 0.5% to 5.0% by weight of caustic.

3. A caustic bath for etching or dissolving aluminum from bodies of aluminum or aluminum-base alloys consisting essentially of an aqueous solution of a caustic etching agent containing sodium lignosulfonate in amount of 0.5%-5.0% by weight of caustic.

4. A solid, substantially water-soluble composition for etching aluminum composed essentially of a caustic agent and containing up to 5.0% of sodium lignosulfonate.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE PROCESS OF ETCHING A BODY OF ALUMINUM COMPRISING CONTACTING SUCH A BODY UNDER ETCHING CONDITIONS WITH AN AQUEOUS SOLUTION OF A CAUSTIC ETCHING AGENT CONTAINING SODIUM LIGNOSULFONATE. 